As is known, air entrapped in a hydraulic system generally has an adverse effect on the control and regulating behavior of the hydraulic system. This air can on the one hand be so-termed primary air, which enters the hydraulic system during the assembly of its components and which, in the ready-assembled unit, undesirably remains trapped at some point or another. On the other hand, air can also make its way into the hydraulic system at a later time, during the operation of the ready-assembled unit.
From DE 19717043 C2 it is known that water present in the hydraulic fluid of a hydraulic unit and gases dissolved in the hydraulic fluid of a hydraulic unit can be separated off by pervaporation methods. In this case the gas or water evaporates at an oil-permeable membrane fitted at a suitable point in the hydraulic system.
DE 19933620A1 describes an oil filter in which is incorporated a semi-permeable membrane for separating off air fractions present in the oil. A semi-permeable membrane of this type is also suitable for venting other dead spaces in which air can or does collect.
DE 4210979 C2 describes a water-tight housing for an electric control unit, which is vented by a semi-permeable wall integrated in a plug. For this a spark-plug is used, whose electric contacts are cast and soldered to a plate inside the housing, whereby the plug together with the housing form a unit which is sealed relative to the outside.
A further possibility known from practice for venting valve-housing components of a hydraulic system immersed in oil is to use venting diaphragms—for example fitted with a sieve-screen insert—at suitable points in the control unit, for example in an outlet duct of a hydraulic valve. Although such a device reliably vents the hydraulic space concerned, this takes place at the cost of a continual oil leak which has to be allowed for in the design of the oil supply to the control unit and generally leads to some undesired loss of performance in the system. Besides, in some operating situations it can happen that air from the outside is drawn back into the hydraulic control unit by way of the venting diaphragm.